The copper containing enzymes peptidylglycine α-hydroxylating monooxygenase (PHM), dopamine β-monooxygenase (DβM), and lytic polysaccharide monooxygenases (LPMO’s) carry out the hydroxylation of strong C–H bonds (85-100 kcal mol-1) in organic substrates. A key intermediate in the catalytic cycles of these enzymes are end-on (η1) mononuclear primary copper(I)-dioxygen adducts, i.e., cupric superoxide (Cu-O-O●–) complexes, which are capable of performing hydrogen atom transfer (HAT) reactions on C-H and O-H bonds. These complexes are thus of great interest to researchers, and many models with steric and geometrical changes in the ligand scaffold have been made to examine Cu-O2 complexes. The (ligand)CuII/(ligand)CuI redox potential has been shown to affect the thermal stability of Cu-O2 species; however little is known about the redox potential effects on the hydrogen atom transfer (HAT) reactivity of the superoxide. To study this effect, while keeping other structural/coordination chemistry properties constant, a series of tris-(2-pyridylmethyl)amine (TMPA) based ligands were synthesized substituting either electron withdrawing groups (NO2 CF3, Cl) or electron donating groups (dimethylmethoxy, dimethylamino) at the para position of the three ligand pyridyl rings. The (ligand)CuII/(ligand)CuI redox potential ranges from -115 mV (NO2TMPA) to -685 mV (DMATMPA) vs Fc+/Fc in acetonitrile as solvent. These complexes all form superoxides, but show large differences in superoxide thermal stability, with DMATMPACu-O-O●– being the most stable. All complexes perform HAT reactions with 2,6-di-tert-butyl-4-methoxyphenol (BDEOH = 80.8 kcal mol-1) but not with 2,4,6-tri-tert-butylphenol (BDEOH = 82.7 kcal mol-1) at -135 °C in MeTHF. The second order rate constant (k2) of this reaction increases linearly with redox potential, with NO2TMPA reacting almost 70x faster than DMATMPA. Insight into the HAT reaction mechanism is found through studying the kinetic isotope effect (KIE) of the reaction as well as through Van’t Hoff and Hammett analysis. Additionally, the dimeric dicopper-dioxygen trans-peroxide species [(L)2CuII2(O2)], formed from the reaction of the superoxide species with CuI, were investigated by low temperature CV and spectroelectrochemisty. Preliminary results show a dependence on the CuII/CuI redox potential on the oxidation of the trans-peroxide species, resulting in a new intermediate with unique behavior.

Redox potential influence on copper-dioxygen adducts and reactivity

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